1. Field of the Invention
The invention relates to the field of impellers for moving fluids axially relative to a rotating shaft. In particular, the invention concerns a hydrofoil impeller having blades of plate or sheet material defining a section of a cylindrical surface, whereby the blades can be formed by rolling operations. The blades are pitched relative to the axis of the cylindrical surface and relative to the radius of the impeller shaft, and encompass a high percentage of the flow cross section.
2. Prior Art
Various forms of impellers are known for generating an axial flow in a fluid with rotation of one or more impeller arrangements on a shaft. The shape of the impeller blade, and in particular the pitch of the blade as a function of radius, can be subjected to mathematical analysis. At shorter radii, the linear velocity of the blade in the fluid is less than at longer radii. Accordingly, the blade is pitched continuously at a steeper angle adjacent to the impeller shaft and at a shallower angle proceeding radially outwardly.
In addition, there is more area available for the impeller blade at larger radii than at smaller radii. The area, pitch, shape and thickness of the blades, and potentially the shapes of the leading, trailing and radial outside edges, are varied to achieve flow characteristics and other attributes that are desirable in a particular application.
An impeller can be cast with blades of complex shape, pitch and varying thickness. However, this results in a heavy structure. Any cast impeller is expensive, and a large cast impeller may be prohibitively expensive. Costs are reduced by producing the blades separately, and attaching the blades to a hub arrangement on the shaft. The blades can be made from solid plates of uniform thickness, bent to approximate the shape needed for the application. The application may require any combination of axial, rotary, tangential and/or radial flow patterns. The present invention, however, concerns an impeller with curved plate blades, optimized for substantially exclusively axial flow.
U.S. Pat. Nos. 4,147,437 --Jonqueres and 4,468,130 --Weetman teach variations on the shape and character of plate blades for axial flow impellers, generally known as hydrofoil impeller blades. Such impellers are useful for mixing and aerating operations, in particular producing a circulating axially downward flow along the center line of a tank, with an axially upward flow around the periphery. Gas may be sparged into the e.g., below the impeller, where the gas bubbles rise against the axially downward flow. One object of impeller blade design is to obtain the greatest efficiency of fluid movement, which efficiency can be expressed as the volume of fluid moved per unit of power expended to rotate the impeller. Another object of impeller design is to reduce the cost of manufacture without adversely affecting the efficiency of the impeller or the attributes of the impeller for use in its particular mixing application.
U.S. Pat. Nos. 5,046,245 and 4,896,971, both to Weetman et al, concern the design and production of high solidity impellers using plate blades. This form of impeller is useful in sparging applications because the blades, as viewed in plan (axially along the rotation axis), occupy a large proportion of the area in which the impeller rotates, and thus present an obstruction to gas bubbles that rise in opposition to the axial flow.
A plate blade construction presents design challenges related to production efficiency and flow efficiency. The blades are cut from flat plates, and must somehow be formed to obtain the necessary pitch variation, wherein the pitch is steeper adjacent the shaft and shallower proceeding radially outwardly. This would appear to require a complex bending operation. Furthermore, it is advantageous if the curved impeller blade can be attached to the shaft of the impeller using a flat element or blade portion that can be bolted or welded simply, for example to a flat connecting arm or the like that protrudes radially from the impeller hub.
In the foregoing Weetman et al U.S. Pat. No. 5,046,245, an attempt to resolve these apparently conflicting design objects is made by forming an isolated bend along a diagonal of a flat blade, thereby providing flat blade portions with a bend between them. The blade is attached to the impeller hub via a flat connecting arm and a backer plate that attach at one of the flat portions by bolts. This solution is advantageous from a production standpoint in that a flat surface is provided for attachment and the bending operation is straightforward. However, the resulting impeller blade is a poor approximation of the optimal hydrofoil shape. Accordingly, the impeller is not particularly efficient with respect to the volume of fluid moved vs. power expended to rotate the impeller.